1. Field of the Invention
The invention relates to a pulse oximeter that not only detects an external noise level affecting measurement of the saturated oxygen in the arterial blood (SPO.sub.2) so that a measurement with the external noise eliminated can be obtained, but also displays an alarm in the case where an accurate measurement cannot be made.
2. Related Art
In the conventional SPO.sub.2 measurement, it is known that the light-receiving element of a measuring probe is affected not only by external light from light sources other than infrared light and red light used in the measurement, e.g., a fluorescent light and the like, but also by induced noise from external devices, e.g., an electric blanket and the like. To overcome this problem, various methods have been proposed, the methods being intended to suppress disturbance affecting a target signal and thereby obtain only the target signal.
FIG. 8 is a block diagram showing a configuration of such a conventional pulse oximeter with part of circuits thereof. In FIG. 8, this conventional example is provided as including light-emitting diodes 2a and 2b and a control circuit 3. The light-emitting diode 2a blinks red (R) light at a frequency f during a first half interval, which is a data sampling cycle T/2, by causing a switching transistor to be turned on and off by a R light emission drive signal. The light-emitting diode 2b blinks infrared (IR) light at the frequency f during the latter half interval, which is a data sampling cycle T/2, by causing a switching transistor to be turned on and off by an IR light emission drive signal. The control circuit 3 controls the emissions so that emission of the R light and the IR light from the light-emitting diodes 2a, 2b is repeated alternately at the frequency f. The control circuit 3 has a CPU, a ROM that stores a control program, a working RAM, and the like.
This conventional example is further provided as having a light-receiving diode 4, a current/voltage converting and amplifying circuit 5, a bandpass filter (BPF) 6, and an AM detecting circuit 7. The light-receiving diode 4 receives transmitted light or reflected light obtained at the time the R light and the IR light from the light-emitting diodes 2a, 2b are irradiated onto the arterial blood, and outputs a photoelectrically converted light-receiving signal. The current/voltage converting and amplifying circuit 5 outputs an amplified light-receiving signal obtained by converting the photoelectrically converted current from the light-receiving diode 4 to a voltage and amplifying the converted voltage. The BPF 6 cuts off low and high frequency ranges of the light-receiving signal from the current/voltage converting and amplifying circuit 5 by a center frequency f. The AM detecting circuit 7 has operational amplifiers and diodes, and outputs a detection signal by detecting (rectifying both waves) the light-receiving signal from the BPF 6.
This conventional example is still further provided as including a switching circuit 8, an integrating circuit 9, and a radio transmission section 10. The switching circuit 8 performs a switching operation in response to an R/IR switching signal from the control circuit 3, the switching operation being such that a light-receiving signal of the reflected light of each of the R light and the IR light can be introduced in the order of irradiation of the R light and the IR light while dividing a single data sampling time (cycle T) into two intervals. The integrating circuit 9 alternately outputs an R signal and an IR signal, each being obtained by integrating the detection signal output from the switching circuit 8 by a constant determined by a capacitor C and a resistor R, based on the R/IR switching signal. The radio transmission section 10 transmits measured data from the control circuit 3.
An operation of this conventional example will be described next.
FIGS. 9(a) to (g) are timing charts showing processed waveforms and the processing timings in the operation of this conventional example. In FIGS. 8 and 9, the R light and the IR light are emitted alternately with the R/IR light emission drive signals shown in FIGS. 9(a) and (b) supplied every cycle T/2 to the light-emitting diodes 2a, 2b through transistors Q1, Q2 from the control circuit 3. The light-receiving diode 4 receives transmitted light or reflected light obtained at the time these emitted lights are irradiated onto the arterial blood, and outputs a photoelectrically converted light-receiving signal. This light-receiving signal is converted to a voltage and the converted voltage is amplified to obtain an amplified light-receiving signal shown in FIG. 9(c) by the current/voltage converting and amplifying circuit 5. Such amplified light-receiving signal is then output to the BPF 6. The BPF 6 passes the amplified light-receiving signal only through a pass band whose center frequency is f.
The amplified light-receiving signal from the BPF 6 is detected by the AM detecting circuit 7. A detection signal shown in FIG. 9(d) is output to a movable contact c of the switching circuit 8. The movable contact c is switched at a cycle of T/2 of the R/IR switching signal shown in FIG. 9(e) from the control circuit 3, and allows the detection signal to be supplied to the integrating circuit 9 through fixed contacts a, b in such a manner as to correspond to the light emission cycles of the R light and the IR light. It is from the integrating circuit 9 that outputs an R signal and an IR signal shown in FIGS. 9(f), (g) obtained by integrating the detection signal by the CR constant. The level of the R signal or the IR signal is equal to the transmitted light or reflected light of the R light and the IR light irradiated onto the arterial blood. That is, such level is equal to an SPO.sub.2 measurement.
In this case, if the light-emitting frequency f of the light-emitting diodes 2a, 2b is increased compared with a conventional frequency so that an amplified light-receiving signal is allowed to pass only the pass band of the BPF 6 (whose center frequency is f), then the pass band can be narrowed to cut off noise in both low and high frequency ranges. That is, many external noises disturbing SPO.sub.2 measurement are distributed in relatively low frequencies that are only several times the ordinary commercial ac power frequency, and these external noises are eliminated by the BPF 6 to thereby improve discrimination of the target signal components (R light and IR light), which therefore allows accurate measurement to be made.
However, while the aforementioned conventional pulse oximeter not only emits the R light and the IR light at a higher light-emitting frequency f than the conventional frequency, but also passes the amplified light-receiving signal only through the pass band (whose center frequency is f) of the BPF 6 to allow discrimination of the external noise from the signal components to be improved, such method is not effective with respect to a diversity of external noises, and in the case where a frequency component close to the light-emitting frequency f is present in a noise, i.e., in the case where the so-called "disturbance" is present, accurate measurement cannot be made.
That is, the frequency component of the external noise close to the light-emitting frequency f is superimposed on the detection signal, which in turn adds an offset voltage to the R signal and the IR signal from the integrating circuit 9. In addition, beat sound due to a phase difference between the frequency f and the light-emitting cycle T is produced as a noise signal.
Under these circumstances, the conventional pulse oximeter displays SPO.sub.2 measurements on a display screen without eliminating the noise. In other words, since the operator cannot identify inaccurate measurements with noise superimposed thereon, no such measures to obtain accurate measurements as changing the measurement site and turning off the power of the noise source can be taken, which has been a shortcoming encountered by the conventional example.